feat: Add presence heartbeat for Matrix online status

- matrix-gateway: POST /internal/matrix/presence/online endpoint
- usePresenceHeartbeat hook with activity tracking
- Auto away after 5 min inactivity
- Offline on page close/visibility change
- Integrated in MatrixChatRoom component

.cursor/worktrees.json

@@ -0,0 +1,5 @@
+{
+  "setup-worktree": [
+    "npm install"
+  ]
+}